Energy Question — Chain sliding off frictionless table

1. The problem statement, all variables and given/known data

Here is the problem that’s confusing me: A frictionless chain of length 2.00m is held with 20.0% of its length hanging over the edge of a table. The chain is then released. Determine its speed the moment the entire chain comes off the table. (Answer = 4.34m/s)

2. Relevant equations

I assumed this was an energy related question and that the equations for Kinetic/Potential energy would apply.

3. The attempt at a solution

I wasn’t really sure how to approach this problem but I tried

PE = KE since at first the chain is just hanging and then at the end the entire thing is off the table.

so I tried PE = KE
mgh = 1/2mv^2
(.8*mass)(g)(1.6m) = 1/2(1*mass)(v^2)

I thought you might only need to use 80% of the total mass of the string for the PE since 20% is hanging off. And I thought h would only be 1.6 since that’s the remaining length of the string that still needed to fall. But I didn’t get the correct answer and I’m probably totally wrong with my thinking but this question confused me :s Any help would be appreciated.

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